What can QED tell us aboutneutron stars and vice versa?

Jeremy Heyl, TheoreticalAstrophysics, Harvard-Smithsonian

Center for Astrophysics

Introduction

• The structure, cooling and observations ofneutron stars probes all four forces inregimes inaccessible to Earthboundexperiments. Neutron stars uniquely probe:– Strong-field general relativity

– Nuclear and neutrino physics at supernucleardensities

– QED in ultrastrong magnetic fields

The Nuclear Equation of State

• Softer equations ofstate result in morecompact stars.– Relativistic effects

– Higher surfacegravity

• Heat capacity andemissivity dependsthe composition ofthe core.

Lattimer & Prakash ‘01

Young Cooling Neutron Stars

Yakovlev et al. ‘02

• The luminosity ofyoung cooling neutronstars is a direct probeof the physics ofultradense matter.

• Is there a quark-gluonphase transition athigh chemicalpotential?

Chakrabarty et al. ‘01

ATMOSPHERE: Magnetic Atoms

ENVELOPE: Anisotropic Heat Conduction

MAGNETOSPHERE: Magnetic Lensing

MAGNETOSPHERE: Gravitational Lensing

JSH, Hernquist; Shaviv, JSH, Lithwick ‘99

Polarized Light near NeutronStars

• The thermal radiationfrom the surface ofneutron stars is slightlyto fully polarized.

• How strongly polarizedis the radiation that wecould observe andwhat can it tell us?

• c.f. Pavlov & Zavlin

PikachuTM is a trademark of the Nintendo corporation. Copyright © 1999 with Nir Shaviv (CITA) andDon Lloyd (CfA)

Contents

• Neutron Stars:– What happens to photons as they pass through

neutron star magnetospheres?

– How does this affect what we see?• Ionized hydrogen atmosphere

– How can we verify that this happens (withtoday’s instruments)?

Neutron Star Atmospheres

• In the atmosphere of a magnetized neutronstar, the opacity of photons in the twopropagating modes may vary by severalorders of magnitude.– κ || ∼ (Eγ/Ecyl) κ ⊥

• The emergent radiation is polarizedperpendicular to the local magnetic field.

• How does it all add up?

The Photon Trajectory

Strong-field QED

• In the magnetic fieldnear a neutron star,many process maybecome important thatwe cannot otherwiseprobe.

• Tracers of theseprocesses aregenerally polarized.

What happens to the polarizationof photons near neutron stars?

• The polarization of light travelling througha transparent medium evolves as:

– s is the normalized Stokes polarization vector

– Ω is the birefringent vector which for QEDpoints in the direction of the projection of themagnetic field onto the Poincaré sphere.

dlds~ = Ω~ â s~

Polarization-Limiting Radii• The polarization modes

of high-energy photonscouple further from thestellar surface thanthose of low-energyphotons.

• Also, if the direction ofthe magnetic fieldchanges appreciablyduring coupling we geta circular component.

1021Hz

1013Hz

1015Hz

1017Hz

Why Is This Important?

• Cheng and Ruderman exploited the plasma-induced low-energy PL radius to explain thehigh net polarization of pulsars in the radio.

• We can do the same here! Except:– We understand the emission in detail.

– The emission comes from the surface.

– Plasma birefringence is familiar; vacuumbirefringence is unprobed.

Low-Energy Polarized Images

Zero Hz - QED neglected 1015 Hz - Optical/UV

High-Energy Polarized Images

1021 Hz - Gamma-Ray1017 Hz - Soft X-Ray

Polarized Spectra from Neutron Stars

101012 12 GG 101014 14 GG

||-mode||-modeQEDQED

||-mode||-modeQEDQED

Total FluxTotal Flux

⊥⊥ -mode QED-mode QED

⊥⊥ -mode no QED-mode no QED

||-mode no QED||-mode no QEDTotal FluxTotal Flux

⊥⊥ -mode QED-mode QED

⊥⊥ -mode no QED-mode no QED

||-mode no QED||-mode no QED

Polarized Fractions

101012 12 GG

101014 14 GG

With QED

Without QED

Implications

• The best diagnostics for understanding NSstructure are in optical/UV not X-ray!

• A detection of the polarized thermalradiation at any energy from a NS willverify the birefringence of the magneticvacuum!

• The polarized fraction depends strongly ongeometry and field strength.

X-ray Prospects

• XPE SMEXfor thecurrentNASA AO

Sensitivitiy

Let’s Do It!

• Optical/UV emission originating from thestellar surface has been detected from:

• Performing the observations as far into theUV as possible reduces the contaminationfrom synchrotron emission plus the objectsare brighter and more polarized in the UV.

RXJ 1856.5-3754RXJ 0720-3125PSR B0656+14PSR B0950+08Geminga

U=24.4B=26.6B=24.98F(130LP)=27.1V=25.3

36ks120ks210ks900ks90ks

Polarizer forACS is only15% efficient at2500Å

Optical Prospects

• Ground-based polarimeters achieve higherthroughput. Prism analyzers can achievenearly fully transmission.

• 24ks on Keck in B would detect 20%polarization from RXJ 0720-3125.

• Polarimetry of these objects is possible(although one would need a generous TAC)using HST or 8-meter telescopes!

What can QED tell us about neutron stars and vice versa?IntroductionThe Nuclear Equation of StateYoung Cooling Neutron StarsPolarized Light near Neutron StarsContentsNeutron Star AtmospheresThe Photon TrajectoryStrong-field QEDWhat happens to the polarization of photons near neutron stars?Polarization-Limiting RadiiWhy Is This Important?Low-Energy Polarized ImagesHigh-Energy Polarized ImagesPolarized Spectra from Neutron StarsPolarized FractionsImplicationsX-ray ProspectsSensitivitiyLet’s Do It!Optical Prospects